1. Field of the Invention
This invention relates to a vibration damping device suitable for use in an automobile suspension system and the like and capable of developing a high vibration damping performance.
2. Disclosure of the Related Art
For example, as this type of the device used for automobile suspension system developing vibration damping performances, some of the inventors have already proposed a vibration damping device in U.S. patent application Ser. No. 07/570,659 (EPC Application No. 90 30 9175.9) filed on Aug. 21, 1990 in order to solve problems inherent to the well-known shock absorber.
In such a device, as shown by a longitudinal section view in FIG. 9, an end portion of each of two cylindrical and flexible membrane members 111, 112 is liquid-tightly connected to respective end portion of a restricted passage constituting member 113 being substantially cylindrical as a whole, while the other end portions of these membrane members 111, 112 are liquid-tightly connected to face plates 114, 115, respectively, whereby a closed chamber 116 is formed in such a manner that folded portions 111a, 112a of these membrane members 111, 112 face to each other. The face plates 114, 115 are connected to each other through a rod-like rigid member 117 inside the closed chamber 116. In the closed chamber 116, water 118 is filled, for example, inside these membrane members 111, 112 and in a restricted passage 119 formed substantially over a full length of the restricted passage constituting member 113. Furthermore, fastening members 121, 122 are arranged onto the face plate 114 and the restricted passage constituting member 113, respectively. The vibration damping device of this structure can be used, for example, by connecting the male screw-shaped fastening member 121 arranged onto the face plate 114 to a body frame of a vehicle through a rubber bushing and also connecting a cup-shaped fastening member 122 arranged onto an outward flange of the restricted passage constituting member 113 to a lower arm of the vehicle through an eye hook 122a.
When vibrations are input to either side of the face plate and the restricted passage constituting member in such a device, the face plates 114, 115 are relatively displaced to the restricted passage constituting member 113 under the flow of water 118 in the restricted passage 119, whereby vibrations can effectively be damped. Further, when the input vibration is a high frequency microvibration, the cylindrical and flexible membrane members 111, 112 are deformed to absorb volume change inside these membrane members, whereby the high frequency microvibration can effectively be isolated.
However, in order to create a large vibration damping force by such a device, it is required to make a sectional area of the restricted passage 119 small because the length of the restricted passage 119 is naturally restricted by the length of the restricted passage constituting member 113. As a result, there is caused a problem that a fear of contacting the structural members with each other becomes high.
Furthermore, there has been proposed a modification of the above device in which an electro-theological fluid is filled in the closed chamber 116 instead of water 118 and the rod-like rigid member 117 and the restricted passage constituting member 113 are acted as positive electrode and negative electrode, respectively, to change over a magnification of damping force in accordance with the intensity of electric field applied. In this case, it is favorable, if the vibration damping force is not required, that it approaches to zero as far as possible, while if the vibration damping force is required, it is made large as far as possible. However, in order to ensure an occurrence of large vibration damping force, there is a problem that when the length of the restricted passage 119 is made long to reduce the sectional area of the passage, the damping force becomes unpractically large even in the absence of electrical field.